Communication system



. Aug. 24, 1943.

Filed Oct. 31, 1941 CARRIER GENERATORS VOICE F RE QUE NC V CHANNEL L-O L-2 VOICE FREQUENCY CHANNEL CARR/ER 'SUPPLV F G. 2 MD :10: Nos

r ZZ rPL T 0/? INPUT lNl/E N TOR 8 y Wh'. THOL DEN A T TORNEV Patented Aug. 24, 1943 UNITED STATES rsrsurorrics COMZMUNICATION SYSTEM William H. T. Holden, Woodside,N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Applicationoctober 31, 1941, Serial No. 417,323

ing being efiective to prevent interference be-.

tween channels without providing individual high frequency channel filters.

According to this invention a desired number of' two-way non-interfering channels are provided in a broad-band high-frequency circuit units to be interconnected. A feature of the invention is the provision of two-way modulator-demodulator units individual to the lines or other low-frequency circuits to be intercon nected. A further feature is an allotment of car rier'frequencies to the required number of channels such that a total band of less than'on octave is utilized for all channels and so that the separation between carr'ier frequencies is not less than three times the voice band to be transmitted,

whereby interference between channels due to intermodulation is prevented.

A clear and complete understanding of the invention will be facilitated by consideration of a system embodying thefeatures of th invention, one such system being represented schematically in the drawing which forms a part of this specification. The invention is, however, not limited in its application to the specific arrangements shown in the. drawing but is generally applicable to systems employing broad-band high-frequency transmission media. Y 7

Referring to the drawing: Fig. 1 discloses ten telephone lines L-I to L-ll,

transmission meditunwithout the use of channel filters by providing means for connecting carrier genera tors of the same frequency to the low-frequency Fig. 2 shows a modulator-demodulator unit MD.

The lines L--l to L0, inclusive, may be subscribers linesor trunks'used in establishing desired telephone connections. Each of the carrier sources Gl to G-5, inclusive, may be any known typeof alternating current source. An alternating generator of the thermionic type is disclosed in the patent to E. H. Colpitts No.

1,624,537 of April 13, 1927. Eachof the low.-fre-- quency filters Fl to F-0, inclusive, is adapted to pass a voice-frequency band from 0 to 3,000 cycles. Reference may be had to the patent to G. A. Campbell No. 1,227,113, granted; May 22, 1917, for disclosure of filters tuned to pass a desiredband of frequencies. Each of the modulator-demodulator units MDI to MDB, inclusive, may be similar to the modulator-demodulator unit MD shown in Fig. 2, which comprises .a full wave rectifier consisting 'of dry type rectify- .ing elements, for instance, copper oxide discs.

The patent to Black No. 1,983,528, December :11, 1934 discloses a modulator-demodulator unit similar to MD and the patent to Griggs No, 1,559,-

inclusive, connected by individual low-frequency filters F-l to F0, inclusive, and individual twoway modulator-demodulator units MD-l to MD-fl to a common broad-band high-frequency transmission medium CM, generators (3-! to G5, inclusive, being provided each for connection to any pair of modulator-demodulator units between which it is desired to establish communication; and

867, November 3, 1925 discloses other modulatordemodulator units which. might be used in practicing my invention. .The aforementioned patent to Black also shows the filter unitrequired between the modulator-demodulator and the voicefrequency line. 1

Referring to Fig. 1, the conductors oftrans mission medium CM are connected to each of the modulator-demodulator units, MDE to MD--0. When a carrier source of the same frequency is connected to any two of the lines L'i to L-0,'a two-way communication path is established between the two lines. As shown in the drawing, the modulator-demodulator MD| of line Ll is connected by Conductors H and [2 to carrier source (3-3 and. the modulator-demodulator MD-6 of line LB is connected by conductors BI and 62 to carrier source G3,v thereby establishing two-way communication between lines L-[ and LB.. The filters F-l ,and F-B prevent the transmission of carrier-frequency currents over the lines L-l and L-B. Modulator-demodulator units MD--3 andMD-9 are-connected, by conductors 3| and 32 and conductors 9| and 92 to carrier source Gl thereby establishing two-way communication between lines L3 and L-9. And modulator-demodulator units MD-E and MD0 are connected, by conductors BI and 82 and conductors land 2 to carrier source G-2 thereby establishing two-Way communication between lines L8 and L-ll. Other connections may be established in like manner, as many simultaneous connections beingv possible a there are carrier sources of different frequencies.

No channel band filters are provided, interference between channels being prevented by a proper allocation of the carrier frequencies. The maximum carrier frequency is less than twice the minimum carrier frequency and the spacing between frequencies may be equal to but not. less than three times the voice band passed by the low-frequency filters. Assuming that the voice band is 4 kilocycles in width, the spacing between carrier frequencies is 12 kilocycles, the minimum carrier frequency becoming 12 n kilocycles, if n is the number of channels required. With a minimum channel frequency of 150'kilo-; cycles, the frequencies of carrier sources G--l, G-Z, G3, G-4 and G-5 for the first five channels would be 150, 162, 174, 186 and 198 kilocycles, respectively. With modulator-demodulator units MD-S and MID- 9 connected to carrier source Gl, side bands of iron-i146 to 154 kilocycles are transmitted through medium CM during conversation between lines and L9. With modulator-demodulator units MD 8 and MD--EI connected to carrier s'ource"G2, side bands of from 158 to 166 k ilocycles are transmitted through medium CM;' and with modula'tor-dm'odulator unitsM-D-i and MID-'6 connected to carrier source G"3', side bands offrom 170 to 178 kilocycles are transmitted through mediuin CM. Interchannel modulation effects will be of a frequency above 4 kil'ocyclesfand will, thereforenot passinto the voic'e fie'quen'cy lines.

In applying the invention to central office' switching, a large numberQof-channel's are required and reference may be had to' the patent to N. Monk No. 2,231,634 February 11', I941 for disclosl'ire of arrangements for providing and connecting carrier sources of desiredfrequency to modulator and clemodulatorunits beingu'sed in establishing connections between lines; i.

Various modifications may be made without departing from the spirit of the invention. For instance the modulater-demodulator units may be of two types, each of the units of one type having fixed carrier frequencies and the units of the other type being adapted to be connected to a carrier source of any desired frequency. Thus the units of one type may be associated with incominglines' or trunks and units of the other type may be associated with outgoing lines or trunks. I

What is claimed is 1. In a telephone system, amultichann'eltrans mission medium; a plurality o f two-way modulator-demodulator units connected to said medium, a plurality of carrier sources, and means connecting the samecarri'er sourceto any two of said un'its' between which communicationis desired, there being as many conversational channels possible at one time as there are car'- riersources of different frequencies, the highest frequency of the carrier sources being less than twice the lowest frequency of the carrier sources and the spacing between carrier frequencies being not less than three times the voice band.

2. In a telephone system, lines, two-way modulator-demodulator units, band-pass filters each adapted to pass voice-frequency currents individually connecting said line'sto said units, a broad-band high-frequency transmission medium constituting a plurality of carrier channels com- 7 mon to saiclunits, carrier sources of different fre-' quencies, one for each channel, and means for connecting each of said units to a carrier source of any desired frequency, the connection of each of any two of said units to a carrier source of the same frequency being effective to establish a two-way communication between the linesto which said twounits are connected, the range] of frequencies used for said channels being less than an octave and the spacing between carrier frequencies beingnot less than three times the filter individually connecting one of saiduni-ts to" oneof said lines, carrier sources of different fr'e quencies, and means for connecting 'ai'carrier source of any desired frequency to any 'of said units, the allocation of frequencies being such that the maximum carrier frequency is less than twice the minimum carrier frequencyand so that the difference between carrier frequencies is not less than three" times the band-passed by each of said filters, said allocation and spacing being effective to prevent interference between 'chan-' nels without providing high-pass channel filters. 4. In a telephone system, 1ines','two-way mod-j ulator-demodulat'or' units. one for each 'of said lines, each of saidJuni ts'i'comprising armrwtvedry type rectifier, -a"b'road -band 'liigh-frequency transmission mediu'mcommon to said "units,

band-pass filters each adapted to pass voi'c frequency currents said filters individually conne'cstingsaid units'to said; lines, carrier sources. of different frequencies, and means for connecting each of said units toa desired oneof said sources, the connection of any two of-said units to sources of the same frequency being'eifective to establish a two-way channel between the lines with which said two units are associated, the allocation of carrier frequencies being such as ,to' make the provision of, channel filters" unnecessary, the maximum carrier frequency being more thantwice the minimum carrier frequency and. the

spacing. between carrier frequencies being not. lessthan three times the band' passed 'by each of said filters. 1 F 1 i WILLIAMHnT. HOLDEN. 

